U.S. patent application number 11/520727 was filed with the patent office on 2007-10-04 for optical signal waveform shaping apparatus.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Shigeki Watanabe.
Application Number | 20070230518 11/520727 |
Document ID | / |
Family ID | 38180283 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070230518 |
Kind Code |
A1 |
Watanabe; Shigeki |
October 4, 2007 |
Optical signal waveform shaping apparatus
Abstract
An optical signal and pump light are input to a nonlinear
optical medium. In the nonlinear optical medium, the optical signal
is amplified with a nonlinear effect caused by the pump light. A
monitor circuit monitors parametric gain in the nonlinear optical
medium. A first power controller increases input power of the
optical signal so that the gain reaches saturation. A second power
controller controls input power of the pump light so as to obtain a
desired gain.
Inventors: |
Watanabe; Shigeki;
(Kawasaki, JP) |
Correspondence
Address: |
BINGHAM MCCUTCHEN LLP
2020 K Street, N.W., Intellectual Property Department
WASHINGTON
DC
20006
US
|
Assignee: |
FUJITSU LIMITED
|
Family ID: |
38180283 |
Appl. No.: |
11/520727 |
Filed: |
September 14, 2006 |
Current U.S.
Class: |
372/22 |
Current CPC
Class: |
H04B 10/299
20130101 |
Class at
Publication: |
372/22 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2006 |
JP |
2006-089434 |
Claims
1. An optical waveform shaping apparatus, comprising: a first power
controller for controlling power of an optical signal; a second
power controller for controlling power of pump light having a
wavelength different from the wavelength of the optical signal; and
a nonlinear optical medium to which the optical signal with power
controlled by said first power controller and the pump light with
power controlled by said second power controller being input,
wherein said first power controller controls the power of the
optical signal so that a gain of parametric amplification by the
pump light is saturated in said nonlinear optical medium.
2. The optical waveform shaping apparatus according to claim 1,
further comprising a monitor unit for monitoring input power of the
optical signal input to said nonlinear optical medium and output
power of the optical signal output from said nonlinear optical
medium, wherein said first power controller increases power of the
optical signal until a gain in said nonlinear optical medium
calculated from the input power and output power of the optical
signal starts decreasing.
3. The optical waveform shaping apparatus according to claim 2,
wherein said second power controller sets the gain at a certain
value by controlling the power of the pump light, and said first
power controller increases the power of the optical signal so that
the gain set at a certain value by said second power controller is
decreased.
4. The optical waveform shaping apparatus according to claim 1,
further comprising: a polarizer provided in a subsequent stage of
said nonlinear optical medium; a first polarization controller,
provided in a previous stage of said nonlinear optical medium, for
controlling a polarization direction of the optical signal to be
orthogonal to a polarization main axis of said polarizer; and a
second polarization controller, provided in a previous stage of
said nonlinear optical medium, for controlling a polarization
direction of the pump light at a certain angle with respect to a
polarization direction of the optical signal, wherein the optical
signal is parametrically amplified in a polarization direction
approximately the same as the polarization direction of the pump
light in said nonlinear optical medium.
5. The optical waveform shaping apparatus according to claim 1,
further comprising an optical band pass filter for transmitting a
wavelength component of the optical signal in a subsequent stage of
said nonlinear optical medium.
6. The optical waveform shaping apparatus according to claim 1,
further comprising a saturable absorber in a previous stage or a
subsequent stage of said nonlinear optical medium.
7. The optical waveform shaping apparatus according to claim 1,
wherein the optical signal is a phase-modulated optical signal or a
frequency-modulated optical signal.
8. The optical waveform shaping apparatus according to claim 7,
further comprising a demodulator, provided in a previous stage of
said nonlinear optical medium, for converting the phase-modulated
optical signal or the frequency-modulated optical signal into an
intensity-modulated optical signal, wherein the intensity-modulated
optical signal from said demodulator is input to said nonlinear
optical medium.
9. The optical waveform shaping apparatus according to claim 1,
further comprising an optical band pass filter for extracting a
wavelength component, which is the same as the optical signal from
output light of said nonlinear optical medium, or a wavelength
component, which is the same as a wavelength of an idler wave
corresponding to the optical signal.
10. The optical waveform shaping apparatus according to claim 1,
wherein the pump light is an optical pulse with a flattened-top
shape.
11. The optical waveform shaping apparatus according to claim 1,
wherein said nonlinear optical medium is an optical fiber, and an
average zero-dispersion wavelength matches or approximately matches
a wavelength of the pump light.
12. The optical waveform shaping apparatus according to claim 11,
wherein the optical fiber is a highly nonlinear optical fiber with
a small mode field.
13. The optical waveform shaping apparatus according to claim 11,
wherein the optical fiber is a highly nonlinear optical fiber
having a core doped with germanium or bismuth.
14. The optical waveform shaping apparatus according to claim 11,
wherein the optical fiber is a photonic crystal fiber.
15. The optical waveform shaping apparatus according to claim 1,
wherein said nonlinear optical medium is a LiNbO.sub.3 waveguide
having a quasi phase matching structure.
16. The optical waveform shaping apparatus according to claim 1,
further comprising: a first polarization controller, provided in a
previous stage of said first power controller, for fixing a
polarization of the optical signal; and a second polarization
controller, provided in a previous stage of said second power
controller, for fixing a polarization of the pump light.
17. The optical waveform shaping apparatus according to claim 1,
comprising in a previous stage of said first power controller: a
splitter for splitting the optical signal into a pair of
polarization components orthogonal to each other; a pair of
polarization controllers for converting the pair of polarization
components into a pair of linear polarizations in the same
direction; and an optical coupler for combining the pair of linear
polarizations, wherein the optical signal output from said optical
coupler is incident to said first power controller.
18. An optical communication apparatus used in an optical
communication system comprising the optical waveform shaping
apparatus according to claim 1, wherein said optical waveform
shaping apparatus shapes a waveform of an optical signal input via
a first optical fiber and outputs the shaped optical signal to a
second optical fiber.
19. An optical waveform shaping method comprising: inputting an
optical signal and pump light having a wavelength different from
the wavelength of the optical signal to a nonlinear optical medium;
monitoring a gain of parametric amplification by the pump light
based on input power of the optical signal input to the nonlinear
optical medium and output power of the optical signal output from
the nonlinear optical medium; adjusting power of the pump light to
obtaina desired gain; and increasing power of the optical signal
until the gain is saturated or decreased.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a waveform shaping
apparatus for, among other purposes, shaping a waveform of an
optical signal being degraded by for example transmission in an
optical fiber.
[0003] 2. Description of the Related Art
[0004] In conventional optical communication systems, the accuracy
of the chromatic dispersion control of a transmission optical fiber
and the characteristics of an optical amplifier amplifying an
attenuated optical signal are primary factors which determine the
limits of transmission speed (bitrate of data) and transmission
distance. A configurations using a transmission line with an
alternating arrangement of a normal dispersion fiber and an
anomalous dispersion fiber or a configuration using a chromatic
dispersion compensator (such as a dispersion compensation fiber)
are well-known technologies for compensating for waveform
distortion caused by long distance transmission. An optical
amplifier (e.g., an optical fiber amplifier) compensates for
attenuation of signal optical power caused by loss in a
transmission fiber.
[0005] Transoceanic systems (e.g. a submarine cable connecting
continents) employing a single wavelength transmission of 10
Gigabit per second (10 Gb/s) or WDM transmission have been
developed, however, there is a current trend of increasing demand
for realizing long distance transmission of high-speed optical
signals (i.e., 40 Gb/s through 160 Gb/s and higher). However, even
with the combination of highly accurate dispersion compensation
technology and a high-quality optical amplifier, optical S/N ratio
is deteriorated by remaining waveform distortion or Amplified
Spontaneous Emission (ASE) noise added by the optical amplifier in
the existing technologies. For that reason, the actual transmission
distance is limited to several hundred kilometers for transmission
speeds of 40 Gb/s, and to only a few kilometers for transmission
speeds of 160 Gb/s.
[0006] Thus, in order to realize a long distance optical fiber
transmission at the high speeds described above, a technology to
shape a distorted waveform and a technology to suppress the
accumulated ASE noise are required.
[0007] Patent Document 1 (Japan Patent 3461121) and Patent Document
2 (Japan Patent 3472151) describe a circuit for shaping a waveform
of an optical signal by employing an optical limiter function. In
the circuits described in the above Patent Documents, a signal
light beam and an auxiliary light beam are inputted to an optical
fiber. Four-wave mixing occurs when optical power of the signal
light beam is higher than a threshold level. In the four-wave
mixing, a portion of the power of the signal light beam is
transferred to the auxiliary light beam, and the power of the
signal light beam is then reduced. By so doing, noise of level "1"
of an optical signal is suppressed, and its waveform is shaped.
[0008] However, although the technologies described in the Patent
Document 1 and the Patent Document 2 have, to a certain extent, a
waveform shaping effect, it is not necessarily sufficient and can
be improved.
SUMMARY OF THE INVENTION
[0009] It is an object of the present invention to provide an
optical waveform shaping apparatus for efficiently shaping a
waveform of a degraded optical signal.
[0010] An optical waveform shaping apparatus comprises a first
power controller controlling the power of an optical signal, a
second power controller controlling the power of pump light having
a wavelength different from the wavelength of the optical signal,
and a nonlinear optical medium into which the optical signal with
power controlled by the first power controller and the pump light
with power controlled by the second power controller are inputted.
The first power controller controls the power of the optical signal
so that the gain of parametric amplification caused by the pump
light is saturated in the nonlinear optical medium.
[0011] The optical signal is parametrically amplified by the pump
light in the nonlinear optical medium. However, if the input power
of the optical signal is increased, the pump light power is
consumed in order to generate four-wave mixing, and the parametric
gain reaches saturation. This saturation enables the realization of
the optical limiter function and suppresses the "1" level noise of
the optical signal.
[0012] The optical waveform shaping apparatus may further comprise
a saturable absorber in a previous stage or a subsequent stage of
the nonlinear optical medium. Using the saturable absorber, "0"
level noise of the optical signal can be suppressed.
[0013] According to the present invention, it is possible to
provide an optical waveform shaping apparatus for efficiently
shaping the waveform of a degraded optical signal. Through the use
of the optical waveform shaping apparatus of the present invention
in an optical communication system, communication quality may be
improved (e.g., improving the optical signal-to-noise (S/N)
ratio).
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram illustrating an operation principle of
an optical waveform shaping apparatus of the present invention;
[0015] FIG. 2 is a diagram illustrating the operations of the
optical waveform shaping apparatus of an embodiment of the preset
invention;
[0016] FIG. 3A and FIG. 3B are diagrams showing a relation between
the optical signal and the pump light;
[0017] FIG. 4A and FIG. 4B are diagrams showing the parametric gain
and the output power of the optical signal with respect to input
power;
[0018] FIG. 5 is a diagram showing the input signal/output signal
when the input power of the optical signal is smaller than the
threshold power;
[0019] FIG. 6 is a diagram showing the input signal/output signal
when the input power of the optical signal is larger than the
threshold power;
[0020] FIG. 7 is a diagram showing a specific configuration of the
optical waveform shaping apparatus of an embodiment of the present
invention;
[0021] FIG. 8A-FIG. 8C are diagrams showing the first example;
[0022] FIG. 9 is a diagram showing the second example;
[0023] FIG. 10 is a diagram showing the third example;
[0024] FIG. 11 is a diagram showing the fourth example;
[0025] FIG. 12 is a diagram showing the fifth example;
[0026] FIG. 13A and FIG. 13B are examples of the demodulator;
[0027] FIG. 14 is a diagram showing the sixth example; and
[0028] FIG. 15 is a diagram showing the seventh example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] FIG. 1 is a diagram illustrating an operation principle of
an optical waveform shaping apparatus of the present invention. In
FIG. 1, an optical signal and pump light (and their optical waves)
are inputted to a nonlinear optical medium 1. Here, the wavelength
of the optical signal input to the nonlinear optical medium 1 is
".lamda..sub.s". The input power of the optical signal is "P.sub.s
(P.sub.s-in)". The wavelength of the pump light is ".lamda..sub.p",
which is different from the wavelength of the optical signal. The
input power of the pump light is "P.sub.p".
[0030] The input power of the optical signal and the input power of
the pump light are controlled by a power controller, which is, for
example, an optical amplifier. The wavelength of the optical signal
output from the nonlinear optical medium 1 is ".lamda..sub.s",
which is the same as that of the optical signal input to the
nonlinear optical medium 1. On the output side of the nonlinear
optical medium 1, an optical band pass filter (for extracting a
wavelength component of the optical signal from the optical output
of the nonlinear optical medium 1) is provided as necessary.
[0031] If the nonlinear optical medium 1 is a third-order or a
second-order nonlinear optical medium, either four-wave mixing
(FWM) or three-wave mixing (TWM) occurs by the pump light in the
nonlinear optical medium 1. This description assumes that the
nonlinear optical medium 1 is a third-order nonlinear optical
medium and that the optical signal is parametrically amplified in a
polarization direction, which is approximately the same direction
as the polarization direction of the pump light by the four-wave
mixing. The gain of the parametric amplification increases
approximately in proportion to the square of the pump light power
P.sub.p (P.sub.p.sup.2), when the pump light power P.sub.p is
sufficiently higher than the optical signal power P.sub.s. However,
if the optical signal power P.sub.s becomes large and the pump
light power P.sub.p is not considered to be sufficiently larger
than the optical signal power P.sub.s, the power of the pump light
consumed to cause the four-wave mixing becomes large. Then, a
phenomenon called "depletion" occurs in which the pump light power
P.sub.p is attenuated in the nonlinear optical medium 1. When the
depletion of the pump light occurs in the nonlinear optical medium
1, the parametric gain in the nonlinear optical medium 1 reaches
saturation drastically (i.e., the gain decreases). As a result,
even if the input power of the optical signal is increased, the
optical signal power output from the nonlinear optical medium 1 is
limited to a certain level, and the parametric amplifier operates
as a so-called optical limiter amplifier. In the optical limiter
amplifier, the output power is constant even when the power of the
input optical signal fluctuates. Therefore the optical waveform
shaping which suppresses intensity noise in which the intensity of
the optical signal is fluctuated (in particular, suppression of "1"
level noise) is realized.
[0032] The nonlinear optical medium 1 is, for example, an optical
fiber. In such a case, the zero-dispersion wavelength of the
optical fiber may match or approximately match the wavelength
.lamda..sub.p of the pump light. This configuration creates a
favorable efficiency of the parametric amplification by the
four-wave mixing. In addition, a configuration employing a highly
nonlinear fiber (HNLF) having the nonlinear optical effect enhanced
as the nonlinear optical medium 1 is effective. The highly
nonlinear optical medium is realized by, for example, an optical
fiber having the nonlinear refractive index enhanced by doping
germanium or bismuth in the core, an optical fiber having the
optical power density enhanced by designing a small mode field, or
a photonic crystal fiber, among other methods. Furthermore, in
other nonlinear optical media, it is possible to employ a
semiconductor optical amplifier with quantum-well structure or a
semiconductor optical amplifier with quantum-dot structure, and so
forth.
[0033] As other devices have the above mentioned parametric
amplification effect (a nonlinear optical medium), a medium may be
used that effectively generates a second-order non-linear optical
effect such as three-wave mixing. In such a case, for example, a
LiNbO.sub.3 waveguide (PPLN) having quasi phase matching structure
or a GaAlAs element can be employed.
[0034] It should be noted that when either the four-wave mixing or
three-wave mixing occurs, the phase of the optical signal is not
distorted in the nonlinear optical medium 1. For that reason, the
waveform shaping method of the present invention is applicable to
such signals as an optical phase modulated signal and an optical
frequency modulated signal in addition to an optical intensity
modulated signal.
[0035] The operations of the optical waveform shaping apparatus of
the present invention are summarized as below:
[0036] (1) An optical signal (signal light beam) and pump light
(control light) each having different wavelengths are input to a
nonlinear optical medium. This generates four-wave mixing or
three-wave mixing in the nonlinear optical medium and the optical
signal is parametrically amplified by the pump light.
[0037] (2) The power of the optical signal input to the nonlinear
optical medium is increased so that the parametric gain in the
nonlinear optical medium either reaches saturation or starts to
decrease. This enables the realization of the optical limiter
operation and suppression of the "1" level fluctuation of the
optical signal.
[0038] (3) By controlling the polarization states of the optical
signal and the pump light, or by employing a saturable absorber,
"0" level fluctuation of the optical signal is also suppressed.
Note that the suppression of the "0" level fluctuation of the
optical signal is not requisite in the present invention.
[0039] The above operations enable the realization of ultrahigh
speed optical waveform shaping with both high efficiency and wide
operating band. Additionally, improvement of the optical S/N ratio
can be achieved.
[0040] In the following description, operations of the optical
waveform shaping apparatus of an embodiment of the preset invention
are set forth:
[0041] FIG. 2 is a diagram illustrating the operations of the
optical waveform shaping apparatus of an embodiment of the preset
invention. This embodiment uses, an optical fiber 11, a third-order
nonlinear optical medium, as an example of the nonlinear optical
medium 1 shown in FIG. 1. For purposes of this embodiment, assume
that the wavelength of the optical signal E.sub.s is
".lamda..sub.s", the frequency is ".omega..sub.s", and the input
power to the optical fiber 11 is "P.sub.s". Further assume that the
wavelength of the pump light E.sub.p is ".lamda..sub.p", the
frequency is ".omega..sub.p", and the input power to the optical
fiber 11 is "P.sub.p". Note that the pump light may be continuous
wave (CW) light, or may be an optical pulse train.
[0042] Both of the optical signal E.sub.s and the pump light
E.sub.p are input to the optical fiber 11. At this time, the
optical signal and the pump light are combined by an optical
coupler and then input to the optical fiber 11. A wavelength
allocation of the optical signal E.sub.s and the pump light E.sub.p
is shown in FIG. 3A. The wavelength .lamda..sub.p of the pump light
may be longer or shorter than the wavelength ".lamda..sub.s" of the
optical signal. The wavelength difference between them is not
limited but, for example, several nm to several tens nm.
[0043] In the interaction between the optical signal E.sub.s and
the pump light E.sub.p, as shown in FIG. 3B, an idler wave E.sub.c
(frequency: .omega..sub.c) which corresponds to the optical signal
E.sub.s is generated by four-wave mixing. The power of the optical
signal E.sub.s and that of the idler wave E.sub.c are amplified
(via parametric amplification) by the four-wave mixing. At this
time, a portion of the energy of the pump light E.sub.p is almost
evenly provided to the optical signal E.sub.s and the idler wave
E.sub.c. Note that the frequencies of the optical signal, the pump
light, and the idler wave should satisfy the following
equation:
.omega..sub.p-.omega..sub.c=.omega..sub.s-.omega..sub.p.noteq.0
[0044] It is given that the length of the optical fiber 11 is "L",
and the loss is ".alpha.". Also given is that the polarization
states of all light waves are equal, and phases match completely in
the optical fiber 11. In addition, the input power P.sub.p(0) of
the pump light E.sub.p is assumed to be sufficiently larger than
the power P.sub.s of the optical signal E.sub.s and the power
P.sub.c of the idler light E.sub.c. In this description,
"sufficiently larger" is defined as ten times larger (10 dB) or
more, for example. Consequently, the optical signal E.sub.s output
from the optical fiber 11 can acquire gain G.sub.s as shown in the
following equation (1)
G S = exp ( - .alpha. L ) [ 1 + .phi. 2 ( L ) ] ( 1 ) .phi. ( L ) =
.gamma. P P ( O ) l ( L ) ( 2 ) .gamma. = .omega. n 2 cA eff ( 3 )
##EQU00001##
[0045] Note that ".phi.(L)" represents nonlinear optical phase
shift, "P.sub.p(0)" represents input power of the pump light,
"l=(1-e.sup.-.alpha.L)/.alpha.L" represents interaction length,
".gamma." represents third-order nonlinear constant, and "n2" and
"A.sub.eff" represents a nonlinear refractive index and an
effective core cross-section area, respectively, in the optical
fiber 11.
[0046] As shown in the above (1)-(3), the optical parametric gain
G.sub.s increases in proportion to squared product of the nonlinear
constant, the input power of the pump light, and the interaction
length. In this example, the generation efficiency of the four-wave
mixing is highly dependent on the polarization state of the
interacting light waves. Specifically, when the polarization states
of light waves inputted to the optical fiber 11 are the same, the
generation efficiency of the four-wave mixing is maximized, but
when the polarization states of the light waves are orthogonal to
each other, the four-wave mixing hardly occurs. Thus, when the
input power of the pump light E.sub.p is sufficiently large, the
optical signal E.sub.s is parametrically amplified selectively in
the same polarization state as that of the pump light E.sub.p.
[0047] In the above model, a condition in which the power P.sub.p
of the pump light E.sub.p is not considered to be sufficiently
larger than the power P.sub.s of the optical signal E.sub.s and the
power P.sub.c of the idler wave E.sub.c is created by increasing
the input power P.sub.s of the optical signal E.sub.s inputted to
the optical fiber 11. As a result, the parametric gain G.sub.s
gradually reaches saturation (or starts to decrease) . In
particular, when in the "depletion" state, in which the pump light
power is reduced in the optical fiber 11 as the power P.sub.p of
the pump light E.sub.p is consumed for generation of the four-wave
mixing, the parametric gain drastically reaches saturation or
decreases.
[0048] FIG. 4A and FIG. 4B are diagrams showing the parametric gain
and the output power of the optical signal with respect to input
power. In this drawing, it is assumed that the input power of the
pump light E.sub.p is constant.
[0049] When input power P.sub.s-in of the optical signal E.sub.s is
small, the parametric gain G.sub.s is constant. Power P.sub.s-out
of the optical signal outputted from the optical fiber 11 becomes
larger as the input power P.sub.s-in increases. Note that in FIG.
4B, a dotted line represents the output power of the optical signal
under the assumption that the parametric gain is zero, and a solid
line represents the output power of the parametrically amplified
optical signal.
[0050] When the input power P.sub.s-in of the optical signal
exceeds a threshold power P.sub.1, the parametric gains G.sub.s
reaches saturation and starts to decrease, as shown in FIG. 4A. The
output power of the optical signal P.sub.s-out approaches the state
represented by the dotted line as shown in FIG. 4B.
[0051] FIG. 5 and FIG. 6 are diagrams explaining the relation
between the gain saturation and waveform shaping. FIG. 5
schematically shows the waveform of the input signal/output signal
when the input power of the optical signal is smaller than the
threshold power P.sub.1. In this case, since the parametric gain in
the optical fiber 11 is not saturated, the output power of the
optical signal is approximately proportional to the input power of
the optical signal. If intensity noise is added to the input
optical signal, the intensity noise is also amplified and
outputted.
[0052] FIG. 6 schematically shows the waveform of the input
signal/output signal when the input power of the optical signal is
larger than the threshold power P.sub.1. Note that in FIG. 6, the
output waveform shown by a dotted line represents output power
under the assumption that the gain was not saturated. The output
waveform shown by a solid line represents output power when the
gain saturation occurs, and it is the actual output waveform of the
optical waveform shaping apparatus of the embodiment.
[0053] The parametric gain in the optical fiber 11 reaches
saturation when the pump light enters the depletion state.
Therefore, the output power of the optical signal is suppressed
compared to the case that operates under the assumption that the
gain was not saturated. In other words, in this case, the optical
limiter function is performed. At the same time, the intensity
noise of the "1" level of the optical signal is also suppressed.
Thus, "1" level noise is removed and the waveform of the optical
signal outputted from the optical fiber 11 is favorably shaped.
Note that the output level in a case in which the input power of
the optical signal exceeds the threshold power P.sub.1 and the gain
is saturated is basically higher than the output level in a case in
which the input power of the optical signal is smaller than the
threshold power P.sub.1 and the gain is not saturated.
[0054] By utilizing the above phenomenon, the intensity noise of
the optical signal may be suppressed. Note that the parametric
amplification is an efficient amplification method which increases
in proportion to the square of the pump light power, approximately.
Thus, a configuration in which an optical signal is processed in
the state in which the parametric gain is saturated is not
favorable in general. In particular, a configuration in which the
pump light is in depletion state has not been employed previously
because gain efficiency is extremely reduced.
[0055] The optical waveform shaping apparatus of the embodiment of
the present invention, however, introduces a configuration in which
strong gain saturation (due to depletion of the pump light) is
employed in order to achieve the optical limiter function. By so
doing, a favorable waveform shaping effect can be obtained.
[0056] It should be noted that the waveform shaping circuits
described in the Patent Document 1 and the Patent Document 2
provide the optical limiter function by utilizing four-wave mixing.
However, those circuits do not use pump light for amplifying the
optical signal, and thus they do not utilize "saturation of gain."
In other words, the circuits reduce (i.e., limit) the power of the
optical signal by transferring the power of the optical signal to
probe light. Therefore, the optical limiter function of the Patent
Documents is weak and the "1" level intensity noise may not be
suppressed sufficiently.
[0057] Alternatively, the optical waveform shaping apparatus of an
embodiment of the present invention operates under the premise of a
configuration in which an optical signal is amplified by using pump
light. The optical limiter function is realized by gain saturation
due to the pump light depletion. The parametric gain, as
represented in the above equations (1)-(3), depends on the square
of an interaction length. Therefore, when the pump light enters a
depletion state, the parametric gain is drastically reduced and
sufficient optical limiter function can be obtained accordingly.
For that reason, the optical waveform shaping apparatus of the
embodiment can perform favorable waveform shaping.
[0058] FIG. 7 is a diagram showing a specific configuration of the
optical waveform shaping apparatus of the embodiment of the present
invention. Note that the nonlinear optical medium 1 is a
second-order or third-order nonlinear optical medium
(.chi..sup.(2)/.chi..sup.(3)) and it can be realized by the above
optical fiber 11, for example.
[0059] A polarization controller 21 controls the polarization state
of an input optical signal. A polarization controller 22 controls
the polarization state of pump light. Note that each of the
polarization controllers 21 and 22 controls the polarization state
of the optical signal and the pump light in accordance with the
instruction from a polarization control circuit 27. For the
polarization controllers 21 and 22, for example, a wavelength-plate
type polarization controller, a LiNbO.sub.3 type polarization
controller, a fiber sqeezing type polarization controller, a
Faraday rotator, and other implements can be used.
[0060] A power controller 23 adjusts the power of the optical
signal having the polarization state controlled by the polarization
controller 21. A power controller 24 adjusts the power of the pump
light having the polarization state controlled by the polarization
controlled 22. Note that each of the power controllers 23 and 24
controls the power of the optical signal and the pump light in
accordance with the instruction from a power control circuit 28.
The power controllers 23 and 24 can be realized by, for example, an
optical amplifier (or a combination of an optical amplifier and an
optical attenuator), among others.
[0061] A monitor circuit 25 monitors the power of the optical
signal inputted to the nonlinear optical medium 1 and the power of
the optical signal output from the nonlinear optical medium 1. Note
that the monitor circuit 25 detects the power of the optical signal
input to the nonlinear optical medium 1 by receiving a part of the
light wave (including the optical signal and the pump light) input
to the nonlinear optical medium 1. In the same manner, the monitor
circuit 25 detects the power of the optical signal output from the
nonlinear optical medium 1 by receiving a part of the light wave
(including the optical signal and the pump light) output from the
nonlinear optical medium 1.
[0062] A comparator circuit 26 calculates a gain in the nonlinear
optical medium 1 based on the input power and the output power of
the optical signal. The comparator circuit 26 compares the input
power and/or output power of the optical signal with a prescribed
threshold, respectively, when necessary.
[0063] The polarization control instruction circuit 27 refers to
the output of the comparator circuit 26, and issues instructions to
adjust the polarization to the polarization controllers 21 and 22.
The power control instruction circuit 28 refers to the comparator
circuit 26, and issues instructions to adjust optical power to the
power controllers 23 and 24.
[0064] It should be noted that the optical waveform shaping
apparatus with the above configuration comprises a pump light
source and an optical band pass filter, however they are not shown
in the drawing. The pump light source generates the pump light (CW
light beam or optical pulses). If the optical pulse is used as the
pump light, a pulse train synchronized with a clock signal
recovered from input optical signal may be generated, for example.
The optical band pass filter is, for example, an optical wavelength
filter having a transmission frequency that is the same as the
wavelength of the optical signal and is provided to the subsequent
stage of the nonlinear optical medium 1. The optical band pass
filter extracts a wavelength component of the optical signal from
the output of the nonlinear optical medium 1.
[0065] In the optical waveform shaping apparatus with the above
configuration, the polarization controllers 21 and 22 and the power
controllers 23 and 24 are adjusted in accordance with the following
procedure.
[0066] First, the states of the polarization controllers 21 and 22
are adjusted. At this time, the polarization states of the optical
signal and the pump light are adjusted so that the nonlinear
optical effect is appropriately generated in the nonlinear optical
medium 1. Note that the nonlinear optical effect (especially,
four-wave mixing) occurs with the maximum efficiency when the
polarization states of the optical signal and the pump light are
identical. Therefore, the polarization controllers 21 and 22 may be
controlled so that the polarization states of the optical signal
and the pump light match each other. The present invention is not
limited to this configuration; the polarization states of the
optical signal and the pump light may be controlled so as to differ
from each other.
[0067] Then the power control instruction circuit 28 adjusts the
state of the power controller 24. In other words, the power of the
pump light is controlled so that the desired or sufficient
parametric gain can be obtained in the nonlinear optical medium 1.
At that time, the input power of the optical signal is small, and
the power of the pump light is significantly larger than the input
power of the optical signal. Specifically, the assumed state is
such that the parametric gain approximately follows the above
equation (1), for example.
[0068] Afterwards, the input power of the optical signal is
increased while monitoring the parametric gain in the nonlinear
optical medium 1. At this point, while the input power of the
optical signal is smaller than the threshold power P.sub.1 shown in
FIG. 4A, the parametric gains is approximately constant. However,
when the input power of the optical signal is increased and becomes
larger than the threshold power P.sub.1, the parametric gain
reaches saturation and starts to decrease. The power control
instruction circuit 28 ends the adjustment of the power controller
23 at the point in time when the parametric gain is reduced by a
prescribed level.
[0069] It should be noted that in the above adjustment procedures,
the extent to which the parametric gain should be reduced depends
on the required level of the optical limiter function (i.e.
waveform shaping function). For example, in a system, in which the
waveform of an input optical signal is estimated to be relatively
favorable, slight reduction of the parametric gain may be
sufficient, putting a high priority on efficiency of the gain
rather than on the waveform shaping effect. On the other hand, in a
system in which the waveform of an input optical signal is
estimated to be considerably degraded, the parametric gain may be
reduced significantly in order to obtain sufficient waveform
shaping effect.
[0070] The adjustment of the input power of the optical signal can
be performed based on the power of the pump light output from the
nonlinear optical medium 1. Here, the optical limiter function for
waveform shaping of the present invention is realized when the pump
light is in the depletion state. Thus, by monitoring reduction of
the power of the pump light output from the nonlinear optical
medium 1 in a process where the input power of the optical signal
is increased while maintaining the input power of the pump light at
a constant level, the waveform shaping can be performed in the same
manner as monitoring the parametric gain.
[0071] In the following description, various usage examples of the
optical waveform shaping apparatus of the embodiment of the present
invention are set forth.
First Example
[0072] In the first example, the optical waveform shaping apparatus
of the present invention is applied to an optical switch having an
amplification function. This kind of optical switch is, for
example, realized by providing a polarizer in the subsequent stage
of the nonlinear optical medium 1 of the optical waveform shaping
apparatus shown in FIG. 7.
[0073] FIG. 8A-FIG. 8C are diagrams explaining operations of the
optical switch of the first example. In the following description,
assume that the optical signal E.sub.s (wavelength: .lamda..sub.s,
peak power: P.sub.s) and optical control pulse E.sub.p (wavelength:
.lamda..sub.p, peak power: P.sub.p) serving as the pump light, are
input to an optical fiber (HNLF) 31 serving as the nonlinear
optical medium.
[0074] In the subsequent stage of the optical fiber 31, a polarizer
32 is provided. A linear polarization component in a direction of
the polarization main axis of the polarizer 32 alone in the optical
signal output from the optical fiber 31 is transmitted through the
polarizer 32.
[0075] The polarization state of the optical signal E.sub.s is
controlled, as shown in FIG. 8A, so as to be orthogonal to the
polarization main axis of the polarizer 32 at the input side of the
optical fiber 31. By so controlling, in a time domain where the
optical control pulse E.sub.p is absent, as shown in FIG. 8B, the
polarization state of the optical signal E.sub.s does not change in
the optical fiber 31. In such a case, the polarization direction of
the optical signal E.sub.s output from the optical fiber 31 is
still orthogonal to the polarization main axis of the polarizer 32.
Hence, in this case, the optical signal E.sub.s is blocked by the
polarizer 32.
[0076] The polarization of the optical control pulse E.sub.p, as
shown in FIG. 8B and FIG. 8C, is controlled so as to be linear
polarization at angle of 40-50 degrees (preferably 45 degrees) with
respect to the polarization of the optical signal E.sub.s. In such
a case, in a time domain where the optical control pulse E.sub.p is
present, the polarization state of the optical signal E.sub.s
changes by influence of the optical control pulse E.sub.p in the
optical fiber 31. At that time, assume that peak power P.sub.p of
the optical control pulse E.sub.p is sufficiently larger than the
power P.sub.s of the optical signal E.sub.s. Then, the optical
signal E.sub.s is parametrically amplified in a polarization
direction of the optical control E.sub.p in the optical fiber 31.
In other words, the optical signal E.sub.s output from the optical
fiber 31 includes a large polarization component in a direction of
the polarization main axis of the polarizer 32. Therefore, in a
time domain where the optical control pulse E.sub.p is present, a
part of the components of the optical signal E.sub.s is transmitted
through the polarizer 32. As above, the optical signal E.sub.s is
switched according to the presence or absence of the optical
control E.sub.p.
[0077] Four-wave mixing (parametric amplification) occurs
selectively for the optical signal having the same polarization
components as the pump pulse. For that reason, when the pump light
power becomes larger to a certain extent, the polarization state of
the optical signal approaches to 45 degree direction, which is the
same as the pump pulse.
[0078] Switching between ON and OFF of the optical signal is
possible without wavelength conversion in the optical switch having
the above configuration. "OFF (zero)" level is blocked to an
adequately low level by the polarizer 32, and at the same time, "ON
(1)" level is output having a large gain by the parametric
amplification effect. Accordingly, a high-performance optical
switch having a high extinction ratio of 30 dB or higher and a
favorable optical S/N ratio can be realized. In the above optical
switch, the polarization direction of the pump light changes by
about 45 degrees with respect to the polarization direction of the
optical signal. Consequently, 3 dB loss is generated in the
polarizer 32; however, by obtaining the parametric gain, which can
be larger than that loss, optical switching with substantial gain
can be achieved. Hence, when the parametric gain is strongly
saturated by increasing the input power of the optical signal to a
level larger than the threshold power, waveform shaping for
suppressing intensity noise of ON level can be achieved. In
addition, fluctuation and noise of the OFF level can be suppressed
by the polarizer 32, and therefore, an optical 2R (Regeneration and
Reshaping) effect can be realized.
[0079] Since the response time of the four-wave mixing in an
optical fiber is extremely high-speed (femtosecond range), the
optical waveform shaping apparatus of the embodiment can shape the
waveform of an ultrahigh speed optical signal of several tera bits
per second (Tb/s).
Second Example
[0080] The optical waveform shaping apparatus of the second example
comprises a saturable absorber 41 in the previous or subsequent
stage of the nonlinear optical medium 1 of the above waveform
shaping apparatus, as shown in FIG. 9. The saturable absorber 41
suppresses the OFF level fluctuation of an optical signal. Note
that the saturable absorber 41 can be realized by a semiconductor
amplifier, a Mach-Zehnder interferometer optical fiber switch, a
nonlinear optical loop mirror (NLOM) switch, or similar means, for
example.
Third Example
[0081] One of the causes of quality deterioration of a
phase-modulated optical signal or a frequency-modulated optical
signal is such that intensity noise (and intensity fluctuation) is
converted into phase noise by the nonlinear optical effect in an
optical fiber. Therefore, suppression of the intensity noise (and
intensity fluctuation) of an optical signal by using the optical
waveform shaping apparatus of the present invention is effective
for quality improvement of a phase-modulated optical signal or a
frequency-modulated optical signal.
[0082] FIG. 10 is an example where the optical waveform shaping
apparatus of the present invention is applied to a communication
apparatus for optical communication system (e.g. an optical
receiver or optical repeater). In this example, a system in which
an optical DPSK signal is transmitted via an optical fiber
transmission path is introduced. Assume that the optical wavelength
of the optical DPSK signal is ".lamda..sub.s".
[0083] In the communication apparatus, an optical DPSK signal and
pump light are input to the above nonlinear optical medium 1. Then,
an idler wave (four-wave mixing light) having the wavelength
.lamda..sub.c is generated by four-wave mixing in the nonlinear
optical fiber 1. Here, phase of the optical DPSK signal (zero/.pi.)
and phase of the idler wave (.pi./zero) are inverted from each
other.
[0084] The optical output (including the optical DPSK signal and an
idler wave) is split by an optical splitter, and one portion is
guided to an optical band pass filter 51 and the other portion is
guided to an optical band pass filter 52. The optical band pass
filter 51 transmits a frequency component .lamda..sub.s. That is,
the optical DPSK signal is obtained from the output of the optical
band pass filter 51. Alternatively, the optical band pass filter 52
transmits a frequency component .lamda..sub.c. That is, an idler
wave signal is obtained from the output of the optical band pass
filter 52. Consequently, by detecting (or demodulating) the optical
DPSK signal or the idler wave, transmitted information can be
recovered.
[0085] It should be noted that FIG. 10 shows a configuration using
a pulse train synchronized with an optical DPSK signal as pump
light; however, the present invention is not limited to the
configuration. A continuous wave beam with high power sufficient to
obtain a desired parametric gain may be used as the pump light.
Fourth Example
[0086] In the configuration similar to that of the third example,
as shown in FIG. 11, an optical pulse train having flattened-top
shape may be used as pump light.
[0087] When generating the four-wave mixing by using the power of
the pump light pulse in an optical fiber, an input optical signal
is applied with an optical phase modulation by cross phase
modulation (XPM). The intensity of the cross phase modulation is
proportional to the peak intensity of the pump light. For that
reason, if the pump light is a short pulse with a large peak, there
is a probability that the optical phase of the optical signal is
disturbed. In view of such a probability, the configuration of the
fourth example uses a pump light pulse train with flattened-top
shape as pump light. In a time domain where the power of the pump
light is constant, influence of the cross phase modulation is
maintained at a constant level, and as a result, the optical phase
disturbance can be suppressed.
Fifth Example
[0088] In an optical receiver (or an optical repeater) receiving a
phase-modulated optical signal or a frequency-modulated optical
signal, as shown in FIG. 12, a demodulator 61 for converting the
optical signal into an intensity-modulated optical signal (i.e.
ON/OFF-modulated optical signal) may be provided in the previous
stage of the optical waveform shaping apparatus of the present
invention. According to this configuration, the phase-modulated
optical signal or the frequency-modulated optical signal is shaped
after being converted into an intensity-modulated optical
signal.
[0089] A demodulator for converting an optical DPSK signal into an
intensity-modulated optical signal, as shown in FIG. 13A, can be
realized by using a 1-bit delay element 62. A demodulator for
converting an optical FSK signal into an intensity-modulated
optical signal, as shown in FIG. 13B, can be realized by using a
band pass filter 63-1 having a transmission frequency f.sub.1 and a
band pass filter 63-2 having a transmission frequency f.sub.2. Note
that the optical FSK signal shown in FIG. 13B is such that each of
the frequencies f.sub.1 and f.sub.2 is assigned to "1" or "0".
[0090] It should be noted that details of the configurations in the
first, third, forth, and fifth examples are described in Japanese
Patent Application No. 2005-200572, which is in the name of the
same applicant as the present patent application.
Sixth Embodiment
[0091] A configuration for waveform shaping without depending on
the polarization state of an input optical signal is shown in FIG.
14. In FIG. 14, a polarization of the input optical signal is fixed
at an arbitrary polarization (e.g., linear polarization or
elliptical polarization) by the polarization controller 21. In
other words, fluctuation of the polarization of the input optical
signal is removed. The power controller 23 controls the power of
the optical signal with the fixed polarization. In the same manner,
a polarization of the pump light is fixed by the polarization
controller 22, and the power is controlled by the power controller
24. The polarization of the pump light may be the same as, or may
be different from, that of the optical signal. Note that control of
the polarization controllers 21 and 22 and the power controllers 23
and 24 is based on the procedures explained with reference to FIG.
7. However, in the sixth embodiment, a polarizer 71 (or PBS:
Polarizing Beam Splitter) is provided between the polarization
controller 21 and the power controller 23. A control system not
shown in the drawing monitors the output power of the polarizer 71.
In such a configuration, the polarization controller 21 may be
adjusted so as to maximize the output power of the polarizer
71.
Seventh Embodiment
[0092] A configuration designed to be independent of the
polarization state of an input optical signal in a method that
differs from that of the sixth example is set forth in FIG. 15.
This optical waveform shaping apparatus avoids dependency on the
polarization state of an input optical signal by using polarization
diversity.
[0093] PBS 81 splits an input optical signal into a pair of
polarization components orthogonal to each other. The pair of the
polarization components is incident to polarization controllers 82
and 83. The polarization controllers 82 and 83 convert each
polarization component into linear polarizations in the same
direction. A optical coupler 84 combines power the pair of linear
polarizations output from the polarization controllers 82 and 83
and directs the combined component to the power controller 23. For
combining the power, an optical circuit 85 for time adjustment may
be provided in order to match the timing of the pair of linear
polarization as necessary. The optical circuit 85 can be realized
by an optical delay element or a Faraday rotator, for example. Note
that controls of the polarization controllers 82, 83, and 22, and
power controllers 23 and 24 are basically the same as the
procedures explained with reference to FIG. 7.
* * * * *